Efficient transposition of IS 911 circles in vitro

An in vitro system has been developed which supports efficient integration of transposon circles derived from the bacterial insertion sequence IS 911 . Using relatively pure preparations of IS 911‐ encoded proteins it has been demonstrated that integration into a suitable target required both the transposase, OrfAB, a fusion protein produced by translational frameshifting between two consecutive open reading frames, orf A and orf B, and OrfA, a protein synthesized independently from the upstream orf A. Intermolecular reaction products were identified in which one or both transposon ends were used. The reaction also generated various intramolecular transposition products including adjacent deletions and inversions. The circle junction, composed of abutted left and right IS ends, retained efficient integration activity when carried on a linear donor molecule, demonstrating that supercoiling in the donor molecule is not necessary for the reaction. Both two‐ended integration and a lower level of single‐ended insertions were observed under these conditions. The frequency of these events depended on the spacing between the transposon ends. Two‐ended insertion was most efficient with a natural spacing of 3 bp. These results demonstrate that transposon circles can act as intermediates in IS 911 transposition and provide evidence for collaboration between the two major IS 911‐ encoded proteins, OrfA and OrfAB.